Radio coupling system



July 19, 1932. H. A. WHEELER RADIO COUPLING SYSTEM Filed June 20. 1929 INVENTOR HAROLD A. wfifafrt ATTORN EYS Patented July '19, 1932 UNITED STATES PATENT OFFICE" HAROLD A. WHEELER, GREAT NECK, NEW YORK, ASSIGNOR "DO HAZELTINE CORPO- RATION, A conPoRA'rIoN or DELAWARE RADIO cournme SYSTEM Application filed June 2t),

This invention relates to high-frequency thermionic amplifiers and more particularly 5 containing capacitive and inductlve imped ance elements associated with the input sections of the successive amplifier stages, the amplification may be caused to vary in a presel-ectedzmanneras the tuning is varied over a desired frequency range.

An object of the invention is to embody in an amplifier having the above characteristics, neutralizing means for minimizing any feedback of energy from the output to the input sections of the individual amplifier stages during operation, which energy transfer usually occurs as a result of the interelectrode capacities of the thermionic tubes and tends to set up sustained oscillations with resultant signal distortion, squeals and whistles.

A second: object of the invention is to provide an amplifier of the type specified adapted for use in radio receiving systems and characterized by the feature that the effective output impedance of the successive amplifier stages is predominantly capacitive over the range of operating frequencies, whereby the effective antenna capacity may be quite simplyadjusted to approximate the capacitive impedances of the successive amplifier stages thereby permitting the use of a coupling network between the antenna and first amplifier stage which is identical both in construction and tuning characteristics with the networks associated with the succeeding stages. I

A further object of the invention is to furnish specific design data for a high-frequency transformer structure the use of which as a part of the coupling networks will result in a high and substantially uniform degree of amplification over a deslred radio frequency range;

Other objects will become apparent from the subsequent detailed description.

It is a well known fact that by interposing between successive amplifier tubes a coupling network having a primary transformer winding across the ouput terminals of the first tube and a tuned secondary winding 1929. Serial No. 372,275.

across the terminals of the succeeding tube,

an amplifier is obtained wherein the overall amplification and also the impedance across the output of the first tube increases as the frequency of tuning is increased. The increased amplification with frequency is un desirable since it-prevents the amplifier from being uniformly sensitive and free fromoscillations over a wide rangev of frequencies. The increase with frequency of the primary impedance is the cause of the corresponding increase of amplification and of feedback currents from plate to grid.

In order to overcome the drawbacks of the above standard type of coupling system, C. E. Trube in application for United States Letters Patent, Serial No. 120,045, filed July 2, 1926, and Patent No. 1,763,380 issued June 10, 1930, has disclosed improved types of couplingsystems, by the proper design of which the amplification of a system may be caused to vary in a preselected manner as the tuning frequency is adjusted over a desired range. These coupling systems are in general characterized bythe additional feature that the total primary impedance connected thereby across the output terminals of a tube may be substantially constant or even decreasing with increased frequency thereby minimizing the tendency for the setting upof sustained oscillations at the higher frequencies due to the regenerative feedback action of the tubes.

To accomplish the above results, the mentioned Trube coupling systems'employ a coupling network one portion ofwhich has a capacitive reactance and another portion an inductive reactance over a desired high-frequency range. The impedance, and hence the voltage across the capacitive element, of course, decreases withincrease in frequency while for the inductive element the same factors increase with frequency. By suitably proportioning the capacitive and inductive elements relatively and suitably coupling the same with the input terminals of the succeeding tube,.it is apparent that the resultant effect on the tube input can be caused to vary Ma preselected manner with frequency over I y and inductive elements be suitably connected across the output terminals of the preceding tube as, for example, by connecting them in series, then the impedance across the tube output can likewise be caused to varyin a desired manner with frequency, for as the impedance of-one element increases, that of the other element. decreases and any desired resultant efl'ect may be obtained.

Despite the improvements introduced by the 'I'rube coupling systems, the amplifica-- tion obtainable is still limited by the feedback action of the tubes at the higher frequencies, due to the fact that the coupling between grid and plate circuits due to the interelectrode tube capacity increases with increase in frequency. The present invention, therefore, embodies in an amplifier utilizing the Trube coupling networks, suitable neutralizing means for minimizing transfers of energy from the ouput to the input of the successive tubes. With such an arrangement all the advantages of the Trube system are available and in addition a much higher degree of amplification over a desired frequency range-is obtainable.

When utilizing high-frequency amplifiers in radio receiving systems, it is desirable that the antenna system have the same impedance characteristics over the operating frequency range as the output impedance 0 the successive amplifier stages, since such an arrangement permits the same type of coupling network to be efiiciently utilized between the antenna and the first amplifier stage as is used to interconnect the successive stages. For effective operation, the coupling networks must be designed to work efficiently between the impedances which they interconnect. Thus, 1f, as is generally the case, the antenna impedance is quite different from the output impedance of the successive amplifier stages, optimum design would call for a different type of coupling network between the antenna and the first amplifier tube than that utilized for interstage coupling.

' Such a nonuniform arrangement is, of course, undesirable from a manufacturing standpoint since cheaper set construction could be obtained with thesame type of coupling networks utilized thruout the receiving set," Also from an operating standpoint, the above described arrangement is undesirable since the first coupling network would tune at a different adjustin position from the others, which would proiibi't the use of unitary control means including gang condensers and the like'for tuning purposes.

The present invention provides a neutraloutput impedances of the successive amplifier stages are capacitive over the operating fre-' quency range, due to the use of a relatively lar e neutralizing condenser connected to eac plate. Thus, by suitably proportioning the antenna structure or associating a fixed capacity of proper magnitude with the antenna circuit, the resultant antenna capacity can be made to approximate the neutralizing capacities of the amplifier stages. As a result, the same type of coupling network may be utilized for coupling the antenna and first tube as is utilized interstage and, furthermore, such networks will be identically tunable, thereby embodying in the amplifier the advantages of uni-controlled tuning pointed out above.

Referring now to the drawing:

Fig. 1 shows a circuit diagram of a complete radio receiving system embodying a uni-controlled tuned high-frequency amplifier constructed in accordance with the present invention.

Figs. 2 and 3 show modified circuit arrangements which are approximately equivalent to the circuit ofFig. 1.

Figs. 4 and 5 disclose the structural details of a special three-windin high-frequency transformer which is suita le for use in the coupling networks of Figs. 1, 2 and 3.

Referring now to Fig. 1, the receiving system comprises thermionic tubes 1 and 2 op erating as amplifiers and tube 3 operating as a detector. Each tube has a grid G, afilament or cathode K constitutingan input section commonly referred to as the grid circuit, and a plate P cooperating with the cathode K to form an output section commonly termed the plate circuit of the tube. The

batteries 4 supply the necessary negative bias on the grids o the amplifier tubes, while the batteries 5 supply the necessary direct cur rent voltages for the plate circuits of the suc cessive tubes. essary energy for heating the cathodes, K.

Associated with' the input to each tube is a colipling network comprising capacities C and 8 together with a three-winding transformer T having a single secondary and two primary windings. The secondary winding L shunted by the variable capacity C constitutes a tunable resonant circuit connected be- .tweengrid and filament of the succeeding tube. The primary circuit connected between plate and filament'of the preceding tube, or to the antenna system terminals 11, comprises the winding L effectively shunted by the fixed capacity C and winding L connected in series with L but in the reverse dimotion. A neutralizing capacity G which serves to neutralize feedback of energy from the plate to the grid circuit of the tube is in each'case connected from the plate to a The capacity C is adapted to tune the sec ondary circuit over a desired frequency Batteries 14 furnish the nec t b t ized high-frequency amplifier wherein the 8 Ween e p'nmary tune the parallel circuit L G to a frequency somewhat lower, usually but slightly lower,

than the lowest frequency-of tuning. Under the above assumption L 6 would be resonant at about 400 kilocycles.- Over the operating range of frequencies the circuit L C thus has a capacitive react-ance which means that the impedance decreases continuously'as the fre quency increases. On the other hand, winding L being substantially purely inductive, its impedance: increases proportionally with increased frequency. Due to the 'smalluinductance and small distributed capacitylofcoil L ,'its resonant frequency is so'much higher than the upper limit of the tuning range of the coupling system, that L may.

over the frequency range, the voltage induced in L by L increases with frequency due to the increasing impedance and hence proportionally greater voltage drop across L On the other hand, the effect produced in L by L decreases with frequency, since as the frequency goes up an increasing percentage of the total primary current flows thru the capacity C In effect, the capacity C shunting L provides a means for automatically reducing the coupling between L and L as the frequency increases.

Thus there are two factors operating on the secondary circuit, one of which increases and the other of which decreases as the frequency of tuning is varied. By suitably proportioning these factors relatively, the resultant effect produced in the secondary circuit can be caused to vary in a preselected manner with frequency. In general, it is desirable to have a design suchthat the over-all amplification of the system remains substantially constant over the operating frequency range. Due to the presence of capacity C ,'the current in winding L is in phase opposition to that in L over the operating frequency range. Hence it is necessary to connect windings L and L series opposed in order to obtain additive effects in winding The winding L in addition to its function as forming part of the primary circuit of the coupling network in the manner explained above,.also cooperates with the capacity C to neutralize the coupling effect occurring between the plate and grid circuits of-a tube as a result of the interelectrode capacity Regenerative feedback occurring between the plate and grid circuits of a thermionic tube, as is Wellknown, is prevented when the circuit arrangements, are such that a voltage active in the plate circuit of the tube can produce no effect in the grid circuit thereof.

"- This [condition is commonly referred to as neutralization. 7 Referring to Fig. 1, if the winding'L is connected in opposite opencircuit polarity to winding L no regenerative feedback will occur when" the following relation obtains f 01 M.-. Where M is thernutual inductance between windings L and -L- Oonsider a voltage active across the-primary circuit associated with the output of tube 2. Such a voltage will cause two capacity currents to flow to ground- The first capacity current flows from plate to rid of tube 2 thru the inter- -electrode coup ing capacity C and thence thru winding L to ground at 9. The second capacity current will flowfrom plate P thru the neutralizing condenser Cg, thru the winding L and in series thru batteries and 4 associated with the tube 1 to ground at 13. If the elements are so pro ortioned that Equation (1) above is satis ed, then the current thus. flowing thru winding L will induse a voltage in winding L, which is equal and opposite to the self-induced voltage in winding L due to the current flowing therein. The resultant induced voltage in winding L will therefore be zero and hence no voltage will be impressed between grid and filament of the tube. 1

In order that the neutralization obtained in this manner be effective at all, fre uencies,

it is necessary that the coupling ietween windings L and L be as close as practicable. The coupling coefficient between these.

.two windings in this case should be about 50% or greater if possible. If the coupling coefficient is too low, the capacity C required for exact neutralization will vary considerably with'frequency.

In order that the amplifier haie a uniform gain over the operating frequency range, it is required that the inductance L be relatively large and L be relatively small as compared to L In practice, L amounts to only a few turns of wire. This means, of course, that the mutual inductance between windings L and L will be quite small as compared to the self-inductance of winding L and if reference be had for a moment to Equation (1.) above, it will be seen that under such conditions the neutralizing capacity C required will be very large as compared to the plate-tov grid interclectrode capacity C In an actual design neutralizing capacities of the, order of 130 lnmf. (micro-microfarads) have been used.

Now referring'for a moment to tube 2 of Fig. 1, it will be noted that the capacity C is connected from the plate P thru the winding L to ground, and is thus effectively in parallel with the output circuit of the tube. Owing to the large value of capacity C its impedance over the operating frequency range is relatively small as compared to the filament-to-plate resistance of the tube and also as compared to the impedances introduced between plate and filament thereof due to the interelectrode tube capacities occurring between the plate and grid, and plate and filament electrodes, all of which impedances are effectively in parallel with the impedance of the neutralizing capacity C In general, the impedance of C over the operating frequency range is about 1000 ohms, as compared to a filament-plate resistance of about 10,000 ohms, a filament-plate interelectrode capacity negligibly small, and agrid.- to-plate interelectrode capacity C of 3 to 10 mmf. The apparent output impedance of the tube is, therefore, approximately that of the capacity C: throughout the operating frequeney range. More accurately, this apparent output impedance of the tube is represented by the total capacity 05+ 0 plus that introduced by leads and wiring. w

The usual capacity of an antenna used with ordinary commercial receiving sets is of the order of 200 mmf. Referring to Fig. 1, by connecting a suitable capacity 8 of say 250 mmf. 'in series with such an antenna 7 having a capacity of about 200 him-1., the resultant capacity of antenna and wiring is of the order of 130 mmf., so that the antenna circuit will have a capacitive impedance over the operating frequency range which simulates the apparent output impedances of the successive tubes as approximately determined by found sufiicientrto provide a single capacity 8 of approximately 250 mmf. If the antenna impedance happens to be high, the capacity 8 is connected in series therewith, and if it happens to be low, the capacity 8 is shortcircuited.

It will become apparent from the discussion given above that the circuit arrangement disclosed in Fig. 1 is very efiicient and economical in that the winding L serves a double purpose, i. e., as a portion of the primary circuit and also as a portion of the neutralizing circuit. Likewise, the capacity C serves a double purpose, i. e., as aportion of the neutralizing circuit. and as a means for furnishing a capacitive output reactance, thereby permitting identically constructed and identically tunable coupling networks to be utilized thruout the receiver.

It has been found that best results are obtained when the coupling between windings L and L is not too loose. The coupling under such conditions might be termed moderate and will be so referred to in the appendedclaims. A moderate coupling, in thispase, is one wherein the coupling coefficient has a value lying between the limits of about 10% and 50%. On the other hand, the coupling between windings L and L should be loose, i. e., having a value less than about 30 percent, while still fulfilling other requirements. In actual coil construction, the above conditions can usually be met due to the fact that the coupling between windings L and L can be as loose as is consistent with the necessary degree of couplings between windings L and L and between windings L and L The ideal condition is attained when the coupling coeificient between windings L and L equals the product of the coupling coefiicients between windings L and L and between windings L and L i. e., when where K represents the coupling coefiicient,

and the subscripts denote the windings bein Fig. 1, and may therefore, be substituted for the arrangement of Fig. 1 either as an antenna coupling system or as an inter-tube coupling system. In Fig. 2, the primary circuit is both inductively and conductively connected to the secondary circuit. The capacity O is connected from the plate of the preceding tube to an intermediate tapping point on L while the winding L is connected in a reversed sense between plate and filament of tube 1 as compared to its mode of connection in Fig. 1. The tapped portion of winding L in Fig. 2 takes the lace of winding L in the primary circuit 0 Fig. 1, and in Fig. 2 the winding L serves as a neutralizing winding only. The circuit of Fig. 2 is electrically equivalent to that of Fig. 1. The only difi'erence consists in combining the separate primary and secondary windings L and L of Fig. 1 into the auto-transformer arrangement of Fig. 2. At the lower frequencies, the capacity C is, in Fig. 2, effectively in shunt with the winding L because the reactance of the tapped portion of L is The negligibly small at such frequencies.

circuit L G; is resonant at a frequency slightly below the minimum fre%uency of-the tuning range of the condenser case for Fig. 1.

Fig. 3 shows acircuit arrangement similar to that of Fig. 2 with the exception that the condenser C is connected from the plate of the preceding tube to the upper terminal of winding L By suitably selectingfthe value of C this circuit may be made electrically equivalent to that of Figs. 1 and 2. In this case the circuit L C L is tuned to a frequency slightly below the tuning range covered by the condenser C. In- Figs. 2 and 3, the capacity C is decreased and the inductance L is increased when the tap on coil L is moved upward to include more turns.

The structural details and design data of a transformer which has been successfully used in the circuit arrangement of Fig. 1 will be given with reference to Figs. 4 and 5. Referring more particularly to Fig. 5, the windings L L and L are of cylindrical form, each comprising a single layer of wire uniformly wound about the tubular sections h, 2' and j, respectively, of insulating material, preferably bakelite. The tubular sections are of successively decreasing diameter as shown, and are positioned coaxially one within another in fixed spacial relationship by means of the bolts n and spacing mem-.

ber m. The secondary winding L being of intermediate diameter is positioned between the primaries with the larger primary L surrounding the same and the small primary L on the inside. I,

The diameter of section 7' is only slightly less than that of i in order to provide the highest practicable coefficient of magnetic coupling between the windings L and L The diameter of section b is considerably greater than that of section 2' in order to provide moderate coupling between the large primary L and the secondary L By positioning the primary windings L and L on opposite sides of the secondary L ,a small degree of magnetic coupling is obtainable between the two primaries, while at the same time providing the necessary coupling between the primaries and the secondary. The loose coupling between L and L is further enhanced by positioning L toward one end of L 1 I There are several advantages in placing thelarge primary L on the outside of windings L and L It will be recalled that the large primary has connected in shunt therewith a capacity of relatively low reactance over the frequency range of tuning. As a result, the Winding L acts in effect as an electrostatic and electromagnetic shield substantially surrounding windings L and L with resultant minimization of the external fields set up by the currents inthese windings and consequent improved operation of the receiving as was the.

system thru reduction of: undesired couplings.

The shielding action of winding L :does not increase the eifective resistance of wind"- ing L :over its value with L on open-circuit, for the reason that with L onopen circuit, the magnetic flux distribution due to a current in winding L is such as to crowd the current in the turns of wire toward the axis of the coil, causing the same to flow thru onlya portion of the conducting cross-sectional area, thereby increasing the total effective resistance in the path of the current flow. With winding'L short-circuited by capacity C3, however, there occurs a redistribution of flux caused by a current flowing 1n winding L since owing to the presence of the shield theflux external to the coil L tends to flow between the coils L and L5. This flux redistribution of L causes the current to be more uniformly distributedover the cross-sectional area of the turns of wire, with resultant decrease in total coil resistance. On the other hand, the magnetic field set up I" by the current in L causes a dissipation of energy in L which appears as an added resistance in L The net efiect, therefore, is that the effective resistance of L is about the same whether L is on open or short Cir-*- cuit, or entirely removed. This same advantage is obtained also when L has the form of a multilayercoil surrounding L As contrasted with the above arrangement, if L were of smallerdiameterv and thus-placed inside of L the total resistance of winding L would be increased because both of the as follows:

a=1 inch 6 242 inches c==.688 inch d= 1.25 inches e=2.5 inches f=1.5 inches g= 1.25 inches cylinders h, i.and jwere each of insulating" material having a wall thickness of Winding L comprised 200 turns of #30 B. & S. gauge enameled copper wire, uniformly wound 80 turns per inch; winding L comprised 126 turns of #26 B. &S. enameled wire, wound 52 turns per inch; while windus A ing L comprised 11 turns of #26 B.& S.

enameled wire, wound 16 turns per inch.

Measurements made at 1,000 cycles to determine the electrical constants of the various windings gave the following results. The self-inductances of the windings measured in microhenries were:

L 1710 mh. L 5.4 mh. L =294 mh.

The mutual inductances were as follows:

M =375 mh. M2-3=25-2 mill. M 18.5 mh.

where M is the mutual inductance, and the subscripts represent the windings between which the mutual inductance is measured as, for example, M refers to the mutual inductance between windings L and L The coeflicients of magnetic coupling between the windings, in percent, were as follows K 53 percent K 26 percent K 46 percent In a radio receiving system of the type shown in Fig. 1 utilizing type UX-QOL-A tubes, and coils of the type shown in Fig. 4 having the constants given above, the follow in values were used for the remaining electrlcal elements. The capacity G was of 250 mmf.;- for the neutralizing capacity G a value of 130 mmf. was found satisfactory; for use in the antenna circuit the fixed capacit 8 had a value of 250 mmf; and the varia le condenser C was continuously adjustable between the limits of 30 and 400 mmf.

I claim: I

1.- The combination with a thermionic tube having input and output sections, of an electrical coupling network having a secondary circuit tunable over a range in frequency connected to said input section, a primary circuit coupled with said secondary circuit and inclu ing an im edance capacitively reactive thruout said requency range, and means independent of said impedance including a portlon of said coupling network for reducing substantially to zero transfers of energy from the output to the input-section of said tube during operation thereof, whereby a uniformly high degree of stable amplification is obtainable thruout said frequency range.

2. The combination with a thermionic tube having input and output sections, of an electrical coupling network having a secondary.

circuit tunable over arange in frequency connected to said input section, a primary circuit coupled with said secondary circuit and. with said output section, said primary circuit including an impedance capacitively reactive thruout said frequency range, and means independent of said impedance but circuit tunable over a range in frequency connected to said input section, a primary circuit coupled to said secondary circuit and to said output section, said primary circuit including a coil resonant at a. frequency below said tunable frequency range, and means independent of said resonant coil, but otherwise including said primary and secondary circuits for reducing substantially to zero transfers of energy from said output to said input section thru-out said range in frequency during operation of said tube.

4. The combination with a thermionic tube having input and output sections, of an electrical coupling network having a secondary circuit tunable over a range in frequency connected to said input section, a primary circuit coupled with said secondary circuit and further coupled thru a neutralizing con denser to said output section, said primary circuit including an impedance capacitively reactive thruout said frequency range, said neutralizing condenser cooperating with said primary and secondary circuits independently of said capacitively reactive impedance to reduce substantially to zero transfers of energy from said output to said input sections thruout said range in frequency during operation of said tube.

5. The combination with a thermionic tube having an input section including grid and cathode electrodes and-an output section including said cathode and a plate electrode, of a tunable secondary circuit associated with said input section, primary circuit comprising a pair of serially connected windings with a capacity connected in parallel with a first said winding and a conductive path between the free end of the second said winding and said cathode, said primary circuit elements being proportioned relatively and said windings being suitably coupled inductively with said secondary circuit to produce upon said input section a combined reaction which varles in a preselected manner as the fre mary circuit comprising a pair of serially connected windings with a capacity connected in shunt with one said winding, said primary circuit elements being relatively proportioned and suitably coupled withsaid secondary circuit to cause'therein a combined reaction adapted to produce a preselected variation in the ratio of output to input energy for said system as the frequency of tuning is varied over said frequency range, one or more of said tubes having a capacity connected from the plate thereof to a po nt between said primary circuit windings of the associated network, said capacity being suitably proportioned and said non-shunted primary winding being suitably poled relative to said secondary circuit for substantially eliminating any tendency for the setting up of undesired oscillations between the output and input of a given such tube due to the coupling effect of the inherent capacity between said, input and output electrodes I thereof.

7. A high-frequency amplifier comprising a thermionic tube having grid, cathode and plate electrodes, a three-winding high-fie quency transformer comprising a secondary and two primary windings, means connecting said secondary winding in shunt with a variable capacity between said grid and said cathode, means serially connecting said primary windings in opposite magnetic polarity, a capacity shunting one said primary winding for resonating the same at a frequency slightly below the tuning range of said variable capacity, a 'conductive path from the free end of the other said primary to said cathode, and a neutralizing capacity connected from a point he tween sai primary windings to said plate and proportioned to minimize transfers of enlergy from-the plate to grid circuits of said tu e.

8. The combination with a thermionic tube having grid and cathode electrodes comprising an input section, and a plate and said cathode electrode comprising an output section, of a high-frequency transformer having a tunable secondary associated with said input section, and inductively coupled therewith a pair of serially connected primary windings, one of which is resonant at a frequency slightly-below the tunable frequency range, and the other of which is effectivelynon-resonant, and a condenser connected from said plate to 3.;P01Ilt .between sald primary windings proportioned to substantially neutralize undesired coupling due to interelectrode capacity of said tube.

, 9. A high-frequency amplifier comprising I a thermionic tube having grid, cathode and plate electrodes, a high-frequency transformer comprising three magnetically coupled windings positioned one within another in fixed, relation, means connecting the intermediate winding thereof shunted byva variable capacity between said grid and cathode, means serially connectingthe outer and in- ,ner windings in magnetic opposition, a capacity shunting said outer winding for resonating the same at a frequency slightly below the tuning range of said variable capacity,

a conductive path betweenthe free end of the inner primary winding and the secondary circuit, and a neutralizing capacity connected from a point between said primar windings to said plate proportioned to minimize transfers of energy from plate to grid circuits of said tube.

10. A high-frequency amplifier comprising a thermionic tube having grid, cathode and plate electrodes, a high-frequency transformer having a tunable secondary winding connected between the grid and cathode, and a pair of primary windings serially connected in magnetic opposition, one said primary winding having a large inducta'nceand the other a small inductance'relative to said secondary winding, a capacity shunting said largeprimary for resonating the same at a frequency slightly below the tuning range of said secondary winding,.said primary windings being suitably proportioned and coupled with said secondary winding to produce, therein a combined reaction which varies in a preselected manner with the frequency of tuning said secondary, and a neutralizing ca pacity connected from said plate to a point between said primaries so proportioned and cooperating with said small primary so poled relative to said secondary as to minimize transfers of energy from plate to grid circuit of said tube.

capacity to said input section, means serially connecting the remaining two windings in magnetic opposition, a capacity shunting said outer windin for resonating the same at a frequency be ow the tuning range of said variable capacity, said windings being suitably proportioned and coupled for impressing upon said input section a combined reaction which varies in a'preselected manner with the frequency of tuning for a voltage impressed across said serially connected windings, said outer winding shunted by said capacity being adapted further to act sub-.

- ings for minimizing transfers of energy from the output to the input sections of said tube.

12. A high-frequency amplifier -eompris ing a thermionic tube having grid, cathode and plate electrodes, a three-windinghighfrequenc transformer having a secondary shunted y a variable tuning capacity connecting said grid and said cathode, a relatively small inductance primary closely coupled magnetically to said secondary, and a relatively large inductance primary moderately coupled 'ma etically to said secondary, a capacity s unting said large primary for resonating the same at a frequency below the frequency range of said tuning ca pacity, means serially connecting said primaries inmagnetic opposition, the proportioning of said primary elements being such as to produce-upon said secondary a com-' bined reaction which varies in a preselected manner with the frequency of tuning, neutralizing means including said small primary and a capacity connected from said plate to a point between said primaries for minimizing transfers of energy from plate to grid circuits of said tube.

13. A high-frequency amplifier comprising a thermionic tube having grid, cathode and plate electrodes, a secondary circuit as necting said grid and cathode, a primary tunable over a desiredfrequency range concircuit containing inductive and capacitive elements suitably proportioned and coupled;

with said secondary circuit to produce therein a combined reaction which varies in a preselected manner with the frequency of tuning, neutralizing means connected between said plate'and cathode'including aneutralizingv capacity in series with a properly poled winding coupled to said secondary circuit for minimizing transfers of energy from plate to grid circuits of said tube, said neuits tralizing capacitybeing adapted further to render the apparent output impedance of said tube capacitive within saidf'frequency range.

14. In combination, .a high-frequency am-- plifier comprising a plurality of thermionic tubeseach having input and output sections, a plurality 'of' identical tunable electrical networks coupling said tubes in cascade, means individual to said tubes for minimizing energy transfers from the output to the input sections thereof, each said means including a relatively large neutralizing capacity effectively in shunt with the output section of the corresponding tube for rendering the apparent output impedance thereof approximately that of said capacity over a desired frequency range, an antenna having sections thereof, each said means including a relatively large neutralizing capacity effectively inshunt with the corresponding tube output section for rendering the apparent output impedance" thereof approximately that of said capacity, vafirst coupling network identical with said plurality associatedwith the input section of the first amplifier tube, and alcapacity associated therewith for adjusting the capacity of an antenna connected thereto to approximate said neutralizing capacity whereby the said first coupling network is identically tunable with said interstage networks. I

16. In combination a thermionic tube having anode, cathode and control electrodes, input and output circuits operatively asso ciated therewith, and means for neutralizing the inter-electrode capacity of said tube including a neutralizing condenser having an impedance relatively small in comparison with the anode-cathode impedance, connected fromsaid anode to a point of said input circuit such that the effective output impedance of said tube is substantially that of said neutralizing condenser.-

17 In combination a thermionic tube havinganode, cathode and control electrodes, input and output circuits operatively associated therewith, and means for neutralizing the inter-electrode capacity of said tube including a neutralizing condenser havingan impedance relatively small in comparison with the anode-cathode impedance, connected from v said anode through a relatively small inductive winding included in said input circuit to saidcathode, whereby the efiective output impedance of said tube is substantially that of said neutralizing condenser.

18. A high-frequency amplifier comprising a-thermionic, tube having anode, cathode and grid electrodes, a secondary circuit tunable over a range in frequency connecting said grid and cathode, a primary circuit containing inductive and capacitive elements suitably proportioned and coupled with said secondary circuit to produce therein a combined reaction which varies in a preselected man- 'ner with the frequency of tuning, neutralizing means connected between said anode and cathode including a neutralizing condenser having an impedance relatively small in comin series with a properly poled small inducl o I \J tive windin coupled to said secondary circuit for mimmizing transfers of energyfrom the anode to the grid circuit of said tube, said neutralizing condenser being adapted further to render the effective output impedance of said tube substantially that of said condenser.

19. In combination, a high-frequency amplifier'com rising a plurality of thermionic I tubes each aving input and output sections,

a plurality of identical tunable coupling cirimpedance thereof substantially that of said condenser over the tunable frequency range,

an antenna circuit adapted to have an effective capacity approximating that of said neutralizing condensers, a coupling circuit identical with said plurality of circuits connecting" said antenna circuit with the input section of the said first tube, and uni-control means for simultaneously adjusting said plur rality of tunable circuits.

20. The combination with a thermionic tube having anode, cathode and grid, of a high-frequency transformer having a tunable secondary connecting said grid and cathode and inductively coupled therewith large and f small serially connected primary windings,

- said large prima I windin being resonant at a frequency s ightly. be ow the tunable range, the small primary winding. being joined at its free ,end to said cathode, and a large neutralizin condenser proportioned to mmimizetrans ers of energy from the anode to the grid circuit of said tube connected from said anode to a point between "saidprimary windin s whereby the effective .ouliput 'impedance 0 said tube is substantia y that of -said neutralizing condenser.

anode to the grid circuitrof said tube, said primary windings being so proportioned and so coupled to said secondary winding as to provide uniformly high sensitivity,'selectivity andstability throughout the tunable frequency range. V

In testimony whereof I afiix m si HAROLD A. W

- ature. ER.

21. Ahi h-frequency amplifier comprisin A a thermiomc tube having anode, cathode an grid electrodes, a three-windinghlgh-fre- 'quency transformer comprising a secondary and two primary windings, means connecting said secondary. windmg in shuntowith a variable tun'ing condenser between said grid and cathode, means serially connecting said primary windings in opposite open-circuit polarity, a capacity shunting one said primary winding for renderin the same resonant at a ir uency below 518 tuning range of said variab e condenser, aconductive path.

from the free end'of the other said primary wlndmg to said cathode, and a neutralizing .condenser connected from said anode to a point between said primary windings for minimizing transfers of energy from the 

